As far as I can remember I don't think I've ever disagreed with you more on a topic than I do on this one, Rol. Allow me to tackle your points one by one...
1) "The new games are too easy."
SMW was even easier. It was the title that introduced an abundance of extra lives and the ability to replay levels. It's also the only game in the series that allows you to start and abort levels while keeping your progress. Need a feather? Go to Donut Plains 1 and get one within seconds. It's also the only game that allows you to store an extra power-up in a box, so you might as well start up Donut Plains 1 twice. It's also the only game that features something like the Top Secret Area where you can stock up on feathers, fire flowers and Yoshis as often as you want. No other game in the series makes it this easy.
As for the difficulty of the levels themselves, the game isn't any harder than either of the NSMB games. The cape also acts like a P-Wing in non-water/non-cave/non-castle levels with vertical scrolling, so about 1/4 of the levels can be finished within seconds. It's not like you'll miss out on anything important.
Archbrix rebuttal: The game's not difficult, I agree (although Starworld provides some nice challenges), but finding all of the secrets/hidden exits and fully exposing the map was the real charm of SMW. Furthermore, I'd argue that obtaining an abundance of extra lives is more satisfying for people having troube than some cheap, cop-out move like Super Guide.
2) "The new games are too short."
The new games contain a few more levels than SMW. The only reason why SMW might have felt like a long game to you is because you were so inexperienced at video games back then when you first played it. Today a 100% rerun of SMW shouldn't take you more than three hours while the NSMB games clock in at around five hours each.
Archbrix rebuttal: I was already a seasoned player when SMW came out and I didn't have any problem at all with the game's length, particularly when achieving the 96 rating (100%).
3) "The new games don't offer enough new things."
SMW was a huge step back from SMB3 in terms of power-ups. SMW is the most disappointing sequel after SMB2, but that game is so bad that it's the worst game in the series in pretty much any category you can think of. SMW also removed competitive multiplayer which came back with NSMBW. What was also added with the Wii game was co-op multiplayer. NSMBU added a challenge mode as well as boost mode. Both NSMB games also featured better power-ups than SMW and the new moves that were first seen in the DS game NSMB, namely double and triple jumps, wall jumps and the ground pound. The latter being a better alternative to SMW's spin jump.
Archbrix rebuttal: The cape > the raccoon tail, IMO, and the statue in SMB3 was useless. Most of the power-ups in games since then just seem more like ways to make Mario look cute rather than be particularly useful. Meanwhile in SMW we get the ability to pick up items and throw them upward; a genius idea that made for clever puzzle solving and a creative final battle. I'm still shocked that they haven't brought this ability back yet. Then there's a veritable smorgasbord of secret exits, Ghost Houses, key finding, and of course, the wonderful overworld map that made SMB3's look like a joke. Oh, and Yoshi! Plenty of new things.
4) "The new games don't push the systems they are on."
Neither did Super Mario World.
Archbrix rebuttal: The game looked fine for a launch game and showed off the SNES' far more robust color palette with some nice sized sprites and some nice scaling/rotation effects here and there, along wih other little things like sounds echoing in caves. No complaints from me.
5) "Super Mario Bros. just isn't what it used to be back then."
It's better this way, trust me. Sonic the Hedgehog's rise happened during the time of Super Mario World. Today Super Mario Bros. thrones above everything and Sonic has been shoved into the downloadable games ghetto.
Archbrix rebuttal: Agreed. Sonic games were incredibly overrated IMO.
6) "Nowadays Nintendo only uses the series to cash in."
And SMW wasn't such a game? The game had to be ready for the SNES launch and was pushed out the door. That's one of the reasons why it barely had any power-ups in comparison to SMB3. Nintendo used the game to sell the SNES.
Archbrix rebuttal: Well, the game certainly didn't feel pushed out the door for launch to me. The level design and characters are still the best in the series, IMHO.
Conclusion: Super Mario World is far from being the flawless and perfect game your memory makes you believe it is. Go back and replay it. Then play NSMBW and NSMBU afterwards and notice the evolution clear as day.
Archbrix conclusion: No, it's not far from what I remeber at all. I've played it many times since the early '90's and I continue to love it each time. SMW remains the best Mario game, 2D or 3D, in existence IMO. Ironically, I believe SMB3, while still great, to be the single most overrated Mario game ever made. The airship levels in SMB3 feel downright uninspired compared to the vastly superior castles in SMW, which show great imagination and still feel fun to this day. Meanwhile, NSMB Wii, to me, is one of the dullest Mario games ever made. In fact, the whole "New" series would have been regarded by me as a bit "meh" if not for the outstanding NSMBU saving the series from such a fate.